Alternate Version of STARTING OUT WITH C++ 4th Edition
Chapter 5Looping
The Increment and Decrement Operators
++ adds one to a variable val++; is the same as val = val + 1;
-- subtracts one from a variable val--; is the same as val = val – 1;
can be used in prefix mode (before) or
postfix mode (after) a variable
Prefix Mode
++val and --val increment or decrement the variable, then return the new value of the variable.
It is this returned new value of the variable that is used in any other operations within the same statement
Prefix Mode Example
int x = 1, y = 1;
x = ++y; // y is incremented to 2 // Then 2 is assigned to x cout << x << “ “ << y; // Displays 2 2
x = --y; // y is decremented to 1 // Then 1 is assigned to x cout << x << “ “ << y; // Displays 1 1
Postfix Mode
val++ and val-- return the old value of the variable, then increment or decrement the variable
It is this returned old value of the variable that is used in any other operations within the same statement
Postfix Mode Example
int x = 1, y = 1;
x = y++; // y++ returns a 1 // The 1 is assigned to x // and y is incremented to 2 cout << x << “ “ << y; // Displays 1 2
x = y--; // y-- returns a 2 // The 2 is assigned to x // and y is decremented to 1 cout << x << “ “ << y; // Displays 2 1
Increment & Decrement Can be used in arithmetic expressionsresult = num1++ + --num2;
Must be applied to something that has a location in memory. Cannot have result = (num1 + num2)++; // Illegal
Can be used in relational expressions if (++num > limit)
pre- and post-operations will cause different comparisons
Introduction to Loops: The while Loop
Loop: part of program that may execute > 1 time (i.e., it repeats)
while loop formatwhile (expression)
{ statement(s); }
If there is only one statement in the body of the loop, the {} can be omitted
No ; goes here
How the while Loop Works
while (expression){ statement(s);}
expression is evaluated if it is true, the statement(s) are
executed, and then expression is evaluated again
if it is false, the loop is exited
while Loop Flow of Control
true
statement(s)
falsecondition
while Loop Example
int val = 5;
while (val >= 0){ cout << val << “ “; val--;}
produces output:5 4 3 2 1 0
while Loop is a Pretest Loop
while is a pretest loop (expression is evaluated before the loop executes)
If the expression is initially false, the statement(s) in the body of the loop are never executed
If the expression is initially true, the statement(s) in the body continue to be executed until the expression becomes false
Exiting the Loop
The loop must contain code to allow expression to eventually become false so the loop can be exited
Otherwise, you have an infinite loop (i.e., a loop that does not stop)
Example infinite loop x = 5; while (x > 0) // infinite loop because cout << x; // x is always > 0
Counters Counter: variable that is incremented or
decremented each time a loop repeats
Can be used to control execution of the loop (loop control variable)
Must be initialized before entering loop May be incremented/decremented
either inside the loop or in the loop test
Letting the User Control the Loop
Program can be written so that user input determines loop repetition
Used when program processes a list of items, and user knows the number of items
User is prompted before loop. Their input is used to control number of repetitions
User Controls the Loop
int num, limit;cout << "Table of squares\n";cout << "How high to go? ";cin >> limit;cout << “\n\nnumber square\n";num = 1;while (num <= limit){ cout << setw(5) << num << setw(6) << num*num << endl;
num++;}
Keeping a Running Total running total: accumulated sum of numbers
from each repetition of loop accumulator: variable that holds running
totalint sum = 0, num = 1; // sum is thewhile (num < 10) // accumulator{ sum += num;
num++;}cout << "Sum of numbers 1 – 10 is " << sum << endl;
Sentinels sentinel: value in a list of values that
indicates end of data
Special value that cannot be confused with a valid value, e.g., -999 for a test score
Used to terminate input when user may not know how many values will be entered
Sentinel Example
int total = 0;cout << "Enter points earned " << "(or -1 to quit): ";cin >> points;while (points != -1) // -1 is the sentinel{ total += points; cout << "Enter points earned: "; cin >> points;}
Using a Loop to Read Data From a File
A Loop can be used to read in each piece of data from a file
It is not necessary to know how much data is in the file
Several methods exist to test for the end of the file
Using the eof() Function to Test for the End of a File
eof() member function returns true when the previous read encountered the end of file; returns false otherwise
Example:datafile >> score;while (!datafile.eof()){ sum += score; datafile >> score;}
Problems Using eof()
For the eof() function to work correctly using this method, there must be a whitespace (space, tab, or [Enter] ) after the last piece of data
Otherwise the end of file will be encountered when reading the final data value and it will not be processed
Using the >> Operation
The stream extraction operator (>>) returns a value indicating if a read is successful
This can be tested to find the end of file since the read “fails” when there is no more data
Example:
while (datafile >> score) sum += score;
The do-while and for Loops
do-while: a posttest loop (expression is evaluated after the loop executes)
Format:do{ 1 or more statements; } while (expression);
Notice the required ;
do-while Flow of Control
statement(s)
condition
false
true
do-while Loop Notes Loop always executes at least once
Execution continues as long as expression is true; the loop is exited when expression becomes false
Useful in menu-driven programs to bring user back to menu to make another choice
The for Loop Top test loop that executes zero or more
times Useful for counter-controlled loop
Format:for( initialization; test; update )
{ 1 or more statements; } No ;
goes here
Required ;
for Loop Mechanics
for(initialization; test; update){ // {} may be omitted statement(s); // if loop body contains} // only 1 statement
1) Perform initialization2) Evaluate test expression
If true, execute statement(s) If false, terminate loop execution
3) Execute update, then re-evaluate test expression
for Loop Flow of Control
true
statement(s)
falsetest
initialization code
updatecode
for Loop Example
int sum = 0, num;
for (num = 1; num <= 10; num++)sum += num;
cout << "Sum of numbers 1 – 10 is " << sum << endl;
for Loop Modifications Can define variables in initialization code
Their scope is the for loop Initialization code, test, or update code
can contain more than one statement Separate with commas
Example:for (int sum = 0, num = 1; num <= 10; num++) sum += num;
More for Loop Modifications (These are NOT Recommended)
Can omit initialization if already doneint sum = 0, num = 1;for (; num <= 10; num++) sum += num;
Can omit update if done in loopfor (sum = 0, num = 1; num <= 10;) sum += num++;
Can omit test – may cause an infinite loop for (sum = 0, num = 1; ; num++)
sum += num;
Deciding Which Loop to Use
while: pretest loop (loop body may not be executed at all)
do-while: posttest loop (loop body will always be executed at least once)
for: pretest loop (loop body may not be executed at all); has initialization and update code; is useful with counters or if precise number of repetitions is known
Nested Loops A nested loop is a loop inside the body
of another loop Example:
for (row = 1; row <= 3; row++){ for (col = 1; col <= 3; col++) { cout << row * col << endl; }}
outer loop
inner loop
Nested Loops Notes Inner loop goes through all its
repetitions for each repetition of outer loop
Inner loop repetitions complete sooner than outer loop
Total number of repetitions for inner loop is product of number of repetitions of the two loops. In previous example, inner loop repeats 9 times
Breaking Out of a Loop Can use break to terminate
execution of a loop Use sparingly if at all – makes code
harder to understand When used in an inner loop,
terminates that loop only and goes back to outer loop
The continue Statement Can use continue to go to end of loop
and prepare for next repetition while and do-while loops go to test and
repeat the loop if test condition is true for loop goes to update step, then tests,
and repeats loop if test condition is true
Use sparingly – like break, can make program logic hard to follow
Using Loops for Data Validation
Loops are the most appropriate structure for
validating user input data1. Prompt and read in the data.2. Use a top test loop to test if data is
valid.3. Enter the loop only if data is not valid.4. Inside the loop, prompt the user to re-
enter the data. 5. The loop will not be exited until valid
data has been entered.
Data Validation Loop Example
cout << “Enter a number (1-100) and” << “ I will guess it. ”;cin >> number;
while (number < 1 || number > 100){ cout << “Number must be between 1 and 100.” << “ Re-enter your number. ”; cin >> number;}// Code to use the valid number goes here.